PERLLOL(1) Perl Programmers Reference Guide PERLLOL(1)
NAME
perllol - Manipulating Arrays of Arrays in Perl
DESCRIPTION
Declaration and Access of Arrays of Arrays
The simplest thing to build is an array of arrays (sometimes impre
cisely called a list of lists). Its reasonably easy to understand,
and almost everything that applies here will also be applicable later
on with the fancier data structures.
An array of an array is just a regular old array @AoA that you can get
at with two subscripts, like $AoA[3][2]. Heres a declaration of the
array:
# assign to our array, an array of array references
@AoA = (
[ "fred", "barney" ],
[ "george", "jane", "elroy" ],
[ "homer", "marge", "bart" ],
);
print $AoA[2][2];
bart
Now you should be very careful that the outer bracket type is a round
one, that is, a parenthesis. Thats because youre assigning to an
@array, so you need parentheses. If you wanted there not to be an
@AoA, but rather just a reference to it, you could do something more
like this:
# assign a reference to array of array references
$ref_to_AoA = [
[ "fred", "barney", "pebbles", "bambam", "dino", ],
[ "homer", "bart", "marge", "maggie", ],
[ "george", "jane", "elroy", "judy", ],
];
print $ref_to_AoA->[2][2];
Notice that the outer bracket type has changed, and so our access syn
tax has also changed. Thats because unlike C, in perl you cant
freely interchange arrays and references thereto. $ref_to_AoA is a
reference to an array, whereas @AoA is an array proper. Likewise,
$AoA[2] is not an array, but an array ref. So how come you can write
these:
$AoA[2][2]
$ref_to_AoA->[2][2]
instead of having to write these:
$AoA[2]->[2]
$ref_to_AoA->[2]->[2]
Well, thats because the rule is that on adjacent brackets only
(whether square or curly), you are free to omit the pointer dereferenc
ing arrow. But you cannot do so for the very first one if its a
scalar containing a reference, which means that $ref_to_AoA always
needs it.
Growing Your Own
Thats all well and good for declaration of a fixed data structure, but
what if you wanted to add new elements on the fly, or build it up
entirely from scratch?
First, lets look at reading it in from a file. This is something like
adding a row at a time. Well assume that theres a flat file in which
each line is a row and each word an element. If youre trying to
develop an @AoA array containing all these, heres the right way to do
that:
while (<>) {
@tmp = split;
push @AoA, [ @tmp ];
}
You might also have loaded that from a function:
for $i ( 1 .. 10 ) {
$AoA[$i] = [ somefunc($i) ];
}
Or you might have had a temporary variable sitting around with the
array in it.
for $i ( 1 .. 10 ) {
@tmp = somefunc($i);
$AoA[$i] = [ @tmp ];
}
Its very important that you make sure to use the "[]" array reference
constructor. Thats because this will be very wrong:
$AoA[$i] = @tmp;
You see, assigning a named array like that to a scalar just counts the
number of elements in @tmp, which probably isnt what you want.
If you are running under "use strict", youll have to add some declara
tions to make it happy:
use strict;
my(@AoA, @tmp);
while (<>) {
@tmp = split;
push @AoA, [ @tmp ];
}
Of course, you dont need the temporary array to have a name at all:
while (<>) {
push @AoA, [ split ];
}
You also dont have to use push(). You could just make a direct
assignment if you knew where you wanted to put it:
my (@AoA, $i, $line);
for $i ( 0 .. 10 ) {
$line = <>;
$AoA[$i] = [ split , $line ];
}
or even just
my (@AoA, $i);
for $i ( 0 .. 10 ) {
$AoA[$i] = [ split , <> ];
}
You should in general be leery of using functions that could poten
tially return lists in scalar context without explicitly stating such.
This would be clearer to the casual reader:
my (@AoA, $i);
for $i ( 0 .. 10 ) {
$AoA[$i] = [ split , scalar(<>) ];
}
If you wanted to have a $ref_to_AoA variable as a reference to an
array, youd have to do something like this:
while (<>) {
push @$ref_to_AoA, [ split ];
}
Now you can add new rows. What about adding new columns? If youre
dealing with just matrices, its often easiest to use simple assign
ment:
for $x (1 .. 10) {
for $y (1 .. 10) {
$AoA[$x][$y] = func($x, $y);
}
}
for $x ( 3, 7, 9 ) {
$AoA[$x][20] += func2($x);
}
It doesnt matter whether those elements are already there or not:
itll gladly create them for you, setting intervening elements to
"undef" as need be.
If you wanted just to append to a row, youd have to do something a bit
funnier looking:
# add new columns to an existing row
push @{ $AoA[0] }, "wilma", "betty";
Notice that I couldnt say just:
push $AoA[0], "wilma", "betty"; # WRONG!
In fact, that wouldnt even compile. How come? Because the argument
to push() must be a real array, not just a reference to such.
Access and Printing
Now its time to print your data structure out. How are you going to
do that? Well, if you want only one of the elements, its trivial:
print $AoA[0][0];
If you want to print the whole thing, though, you cant say
print @AoA; # WRONG
because youll get just references listed, and perl will never automat
ically dereference things for you. Instead, you have to roll yourself
a loop or two. This prints the whole structure, using the shell-style
for() construct to loop across the outer set of subscripts.
for $aref ( @AoA ) {
print "\t [ @$aref ],\n";
}
If you wanted to keep track of subscripts, you might do this:
for $i ( 0 .. $#AoA ) {
print "\t elt $i is [ @{$AoA[$i]} ],\n";
}
or maybe even this. Notice the inner loop.
for $i ( 0 .. $#AoA ) {
for $j ( 0 .. $#{$AoA[$i]} ) {
print "elt $i $j is $AoA[$i][$j]\n";
}
}
As you can see, its getting a bit complicated. Thats why sometimes
is easier to take a temporary on your way through:
for $i ( 0 .. $#AoA ) {
$aref = $AoA[$i];
for $j ( 0 .. $#{$aref} ) {
print "elt $i $j is $AoA[$i][$j]\n";
}
}
Hmm... thats still a bit ugly. How about this:
for $i ( 0 .. $#AoA ) {
$aref = $AoA[$i];
$n = @$aref - 1;
for $j ( 0 .. $n ) {
print "elt $i $j is $AoA[$i][$j]\n";
}
}
Slices
If you want to get at a slice (part of a row) in a multidimensional
array, youre going to have to do some fancy subscripting. Thats
because while we have a nice synonym for single elements via the
pointer arrow for dereferencing, no such convenience exists for slices.
(Remember, of course, that you can always write a loop to do a slice
operation.)
Heres how to do one operation using a loop. Well assume an @AoA
variable as before.
@part = ();
$x = 4;
for ($y = 7; $y < 13; $y++) {
push @part, $AoA[$x][$y];
}
That same loop could be replaced with a slice operation:
@part = @{ $AoA[4] } [ 7..12 ];
but as you might well imagine, this is pretty rough on the reader.
Ah, but what if you wanted a two-dimensional slice, such as having $x
run from 4..8 and $y run from 7 to 12? Hmm... heres the simple way:
@newAoA = ();
for ($startx = $x = 4; $x <= 8; $x++) {
for ($starty = $y = 7; $y <= 12; $y++) {
$newAoA[$x - $startx][$y - $starty] = $AoA[$x][$y];
}
}
We can reduce some of the looping through slices
for ($x = 4; $x <= 8; $x++) {
push @newAoA, [ @{ $AoA[$x] } [ 7..12 ] ];
}
If you were into Schwartzian Transforms, you would probably have
selected map for that
@newAoA = map { [ @{ $AoA[$_] } [ 7..12 ] ] } 4 .. 8;
Although if your manager accused of seeking job security (or rapid
insecurity) through inscrutable code, it would be hard to argue. :-) If
I were you, Id put that in a function:
@newAoA = splice_2D( \@AoA, 4 => 8, 7 => 12 );
sub splice_2D {
my $lrr = shift; # ref to array of array refs!
my ($x_lo, $x_hi,
$y_lo, $y_hi) = @_;
return map {
[ @{ $lrr->[$_] } [ $y_lo .. $y_hi ] ]
} $x_lo .. $x_hi;
}
SEE ALSO
perldata(1), perlref(1), perldsc(1)
AUTHOR
Tom Christiansen
Last update: Thu Jun 4 16:16:23 MDT 1998
perl v5.8.8 2008-04-25 PERLLOL(1)
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